Currently, design and development of circuits or the like by using the computer simulation are widely carried out. Although it is assumed that the simulator realizes actual values in the circuits or the like by the calculated output values, errors occur, actually. For example, as depicted in FIG. 1, in a graph whose horizontal axis represents an input variable value inputted to the simulator and whose vertical axis represents an output variable value from the simulator, it is assumed that a curve “a” in case where “10” is set to an input parameter, which is set separately from the input variable, and a curve “b” in case where “20” is set to the input parameter are obtained. On the other hand, when actual values for specific input variable values are plotted as diamond-shaped points, a curve represented by the diamond-shaped points is almost identical with the curve “b” in case where “20” is set to the input parameter in this example. However, when “10” is set to the input parameter, the curve “a” represented by the calculated values is far from points of the actual values. Thus, it is preferable that the errors are minimized by adjusting the input parameter values for the simulator.
More specifically, a case is considered where Simulation Program with Integrated Circuit Emphasis (SPICE) is used to carry out simulation of the electric current and voltage characteristic of transistors. In such a case, as depicted in FIG. 2, it is assumed that model parameters of the Berkeley Short-Channel IGFET Model (BSIM), which is well-known as the input parameters, are adopted as the input parameters, voltage, gate length and gate width are adopted as the input variables, and when those values are inputted to the SPICE simulator, values of the electric current, which is an output variable, are outputted.
In such a case, for certain input parameter values, a relation between values of the voltage, which is the input variable, and values of the electric current, which is the output variable, is obtained as results of the simulation, as depicted in FIG. 3. On the other hand, FIG. 4 is obtained when relations between specific voltage values and actual values of the electric current are plotted additionally to FIG. 3. Namely, errors represented by the lengths of the arrows occur.
Conventionally, by using, as an error function, Σ(calculation value−actual measurement value)2*weight, the input parameter values are adjusted so as to minimize the value of this error function. Incidentally, the user adjusts the weight to carry out fitting in conformity with his or her sense. However, it is difficult to appropriately set the weight, and even when the weight is set once, the input parameter values have to be adjusted again. Furthermore, the setting of the weight depends on the user's skill, and it is difficult to indicate the objective basis.
Incidentally, some conventional techniques are known for the setting of the input parameters. For example, in a conventional technique, second-order model parameters are generated by carrying out fitting to actual operational terminal voltage condition of devices such as transistors included in the designed circuits and the circuit simulation is carried out by using the second-order model parameters. However, the weight is not considered in this conventional technique.
In addition, another technique exists as follows: Namely, a device model expression and initial values of device parameters are inputted. Then, by carrying out calculation based on the initial values of the device parameters stored in a storage device storing the inputted device model expression and the initial values of the device parameters, and desired voltage-electric current characteristic of the transistors, first to fourth voltages are calculated respectively. The first voltage is a voltage of a source region edge adjacent to a gate electrode edge of a gate electrode side surface on a polycrystalline silicon layer in the transistor, the second voltage is a voltage of the source region edge on an isolated substrate side surface on the polycrystalline silicon layer, the third voltage is a voltage of a drain region edge adjacent to the gate electrode edge on a gate electrode side surface on the polycrystalline silicon layer in the transistor, and the fourth voltage is a voltage of the drain region edge on the isolated substrate side surface on the polycrystalline silicon layer. Furthermore, by substituting these first to fourth voltages into the device model expression stored in the storage device, the drain electric current value is calculated. Then, by comparing the desired voltage-electric current characteristic stored in the storage device with the voltage-electric current characteristic based on the drain electric current obtained by the aforementioned calculation, and changing the device parameters until the difference becomes equal to or less than the allowable error, the model parameters are obtained. However, the weight is not considered in this conventional technique.
Furthermore, a technique exists that a parameter adjusting processing of a physical model including plural parameters is automatically carried out by using Genetic Algorithm. However, the weight used in the error function is not considered in the conventional technique.
Namely, the conventional techniques cannot set the input parameter values for the simulation, appropriately, and cannot evaluate the error between the calculation value by the simulation and the actual measurement value.